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Vol. VIII. APRIL, 1921 No. 4 





The morphology and anatomy of Rhus diversiloba .... James B. McNair 179 

Distribution of the Malvaceae in southern and western Texas 

Herbert C. Hanson 193 

Note on the histology of grain roots Grace A. Dunn 207 

North American pipers of the section Ottonia William Trelease 312 

Monocarpy and pseudomonocarpy in the Cycadeoids . . . . G. R. Wieland 218 




At 41 North Queen Street, Lancaster, Pa, 

Entered as lecond-class matter February 21, 1914, at the post office at Lancaster, Pennsylvania, 
under the act of March 3, 1879 

180 american journal of botany [vol. 8 

The Leaf 

The glossy, dark green leaflets are deepest in color when in the sun, pale 
underneath, generally 3 in number, although sometimes 5, orbicular to 
ovate or oblong-ovate, undulate or plane, entire or variously lobed, seg- 
mented or toothed, 1 to 4 inches long. The 5-leaflet variety, according to 
Brandegee (3) is quite common on the Santa Barbara Islands. Leaves 
having 5 leaflets are also found on plants which have a majority of the 
3-leaflet kind. Leaflets are singularly variable in size, outline, and seg- 
mentation, even on the same plant. This fact constitutes one of the most 
remarkable features of the plant and is the principal basis for its differen- 
tiation horn Rhus Toxicodendron L. Leaf tracings (21) made from mature 
leaves collected by the writer at Berkeley, California, on September 27, 
191 6, were taken from plants within a radius of 100 feet, all of which were 
enjoying the same soil and exposure and had no apparent cause for such 
marked differences in leaf shape. 

Leaves in the sun differ from those in the shade, not only as regards 
color but also in several structural details. The young leaves are covered 
with hairs, which dry out and fall off as the leaves become fully matured. 
These hairs are apparently more frequent on leaves exposed to the sun 
than on those in the shade. Other differences will be described later. 

In autumn, as in spring and summer, the plant is singularly attractive, 
its leaves turning many shades of red, yellow, and brown. This color 
change may be induced in mature leaves in midsummer by certain insect 
injuries, by attacks of fungi, or by an interference with the flow of sap 
caused by twisting the stem. There is no apparent difference between 
the leaves of male and female plants in this respect. Some plants, however, 
particularly those in the shade, may have all their leaves yellow. Con- 
versely, red leaves seem to be peculiar to plants of sunny exposure, although 
there are many exceptions; far more frequently the leaves are mixtures of 
all three colors. The oldest leaves often assume autumnal tints first. 

The petiole in transverse section (21) has in form nearly a semi-circle 
for its dorsal side and a small concave arc as a ventral surface. Under the 
epidermis lie two or three layers of collenchyma cells. The vascular bundles, 
of which there are more than 18, are arranged in a flattened circle parallel 
to the outer surface of the petiole. The pith consists of large, thin-walled 
cells with very small triangular intercellular spaces. The vascular bundles 
are separated from each other by broad medullary rays. Large resin ducts 
are found in the phloem. The primary cortex is bordered internally by a 
starch sheath. The cells of the xylem have thick and lignified walls. The 
pith is enclosed by bast fibers and xylem and takes up the largest part of 
the section. There are no resin ducts in the pith or in the primary cortex. 

The leaf in transverse section exhibits palisade parenchyma occupying 
about one third of the entire thickness of the mesophyl (PI. Ill, fig. 4). 
The spongy parenchyma occupies about five layers of cells. Cells with 


crystal clusters, presumably of calcium oxalate, occur in the palisade 
parenchyma. The cells of the lower epidermis are similar to those of the 
upper epidermis but smaller; stomata are very frequent and apparently 
absent from the ridges. The leaves wilt very easily. It is hardly possible 
to bring a cut branch from the field to the laboratory without observing 
wilting. There are two kinds of trichomes on the leaves, multicellular 
club-shaped, and unicellular or multicellular bristle-shaped (21). 

The thick-walled bristle hairs occur mainly on the lower side on the 
ridges, large and small, of the leaf, although they are found also in fewer 
numbers on the upper side in corresponding places. The club-shaped 
trichomes, on the other hand, are found mostly between the ridges of the 
leaves. These two different forms of trichomes are similar to those found 
by Mobius (22) on Rhus vernicifera L. and by Rost and Gilg (24) on Rhus 
Toxicodendron L. Morphologically the club-shaped hairs seem to be 
glandular: first, because the upper multicellular portion is sharply marked 
off from the basal portion, which resembles a stalk; second, the upper 
portion has thinner walls than the basal portion; third, they are found mostly 
on the young, rapidly growing organs of the plant, especially the floral 
region and the leaves, less on the green stem, and hardly at all on the 
woody portion. Schwalbe (25, 26) considered the poison of Rhus diver siloba 
to be excreted from glandular hairs on the surface of the plant. That such 
is not the case can be shown by the two following experiments : 

(1) When the green stem, pedicel, or main ribs of the leaf, which are 
covered with trichomes, are rubbed on sensitive skin, no dermatitis results. 
Care must be taken, however, that the epidermis of the plant is not broken 
severely enough to cause the resinous sap to exude. 

(2) The fresh green leaves were placed in a finger bowl and soaked at room 
temperature in 95 percent alcohol for 10 minutes. The leaves had been 
examined first under a hand lens to make sure that through possible injury 
no resinous sap was on the surface. When placed in the finger bowl the 
sap was prevented from running down the pedicel from the cut end into 
the alcohol. The leaves when taken out of the alcohol had lost their gloss. 
The pale yellowish alcoholic solution remaining was concentrated by boiling 
in an open beaker. It was found to be non-toxic. It was not darkened by 
potassium hydroxide, nor did it respond to other chemical tests for the 
poison. These results indicate that neither the plant trichomes nor their 
exudate is poisonous. 

The club-shaped hairs are so minute as to be hardly discernible by the 
naked eye. They have a length of 0.071 mm. and a maximum breadth of 
0.0027 mm. Under the microscope they exhibit a clear, unicellular basal 
portion as an outgrowth of an epidermal cell, above which are the numerous 
cells that go to make up the main portion of the hair. The cells of the main 
portion when viewed transversely radiate from a longitudinal central axis. 
The apex terminates in a single cell, and the entire main portion of the hair 


is enclosed in a thin-walled sac. The hairs appear to be of two types, which 
apparently correspond to different stages in development : a densely granular 
and a sparsely granular form. This difference in granular density is 
interesting. In animal glands it has long been noticed that when a serous 
gland has been quiescent for several hours the secretory cells are granular 
throughout, and the outlines of the cells are only faintly marked as clear 
lines bounding the granular areas. When the gland secretes, many of the 
granules disappear and after prolonged secretion very few granules are left; 
i.e., during secretion the granules normally contained by the cells are in 
some way or other used up, probably to form a part of the secretion. Al- 
though the diminution of zymogen granules is a normal occurrence in the 
secretion of the salivary, infra-orbital, lachrymal, mucous, and pancreatic 
glands, yet in the case of the mammary glands the opposite is true, viz., 
that granules begin to form with the commencement of secretion and do 
not occur during rest. In the mammary gland, the active growth of proto- 
plasm, the formation of granules from the protoplasm, and the discharge of 
these granules in the secretion appear to go on at one and the same time. 
Investigation of the club-shaped hairs of Rhus diver siloba has not as yet 
revealed a positively glandular nature, and consequently a relation between 
differences in their granulation can not be definitely connected with secretion. 
From a morphological standpoint, however, as above pointed out, the 
club-shaped hairs seem to be glandular. 

Club-shaped hairs from leaves gathered in the morning before sunrise 
and from those secured in the heat of the day could not be differentiated. 
Hairs from rapidly growing leaves could not be distinguished from those 
of old leaves or stems. Hairs from leaves grown in sunny exposures ex- 
hibited no differences, although they were present in greater number than 
on leaves continuously in the shade. 

The Stem 

A transverse section of a green stem of Rhus diver siloba shows, beginning 
at the outside, the following tissues (PI. Ill, fig. 5): epidermis, with its 
trichomes and stomata; collenchyma; cortical parenchyma; pericycle, with 
bast fibers and thin-walled pericycle parenchyma; phloem, with resin ducts; 
cambium; xylem; medullary ray; pith. 

As the stem increases in diameter (fig. 3) the cortex develops a phellogen. 
The continuous activity of the phellogen results in an increasing thickness 
of the sheet of cork. The chloroplast-containing tissue beneath the cork 
layer maintains connection with the air by means of lenticels which have 
replaced the stomata. As may be anticipated, the dead cork cells are 
non-poisonous, i.e., they do not cause dermatitis when rubbed on the skin 
of a susceptible individual and therefore do not constitute a means of 
transference for the poison. 

No resin ducts have been found in the pith of this plant. Engler (5), 


studying Rhus Toxicodendron L., and Inui (10), studying R. Toxicodendron 
var. radicans, were unable to discover resin ducts in the pith. Jadin (11) 
cited 18 species of the genus which are provided with permanent pith resin 
ducts, and 9 species which do not have them. 

At the periphery of the pith the small outer cells acquire a thick wall 
and become sclerenchymatous. These thick- walled cells may assist the 
inner large-celled and the outer small-celled pith to maintain a circular 
outline. A semi-circular row of bast fibers lies external to the primary 
phloem and serves mechanically to protect the phloem with its resin ducts 
from external injury. 

In the phloem of the second year, new resin ducts appear. These lie 
neither in radial nor in tangential rows, but are so arranged as to be very 
nearly equidistant. The first appear in the secondary phloem between 
two primary resin ducts, and more are formed in a corresponding manner. 
It must not be forgotten, however, that the formation of resin ducts does 
not occur in a regular manner. 

New bast fibers do not appear to be formed in the pericycle. The 
epidermis has been almost wholly lost in the second year and is replaced 
by cork. 

The histology of the pith, wood, and bark of the older stems will be 
treated individually. 

The pith cells are polygonal and lie close together; they are generally 
wider than high, so that their vertical measurement is the smallest. In the 
specimens examined, the pith cells contained for the most part no particular 
substance; starch was found sparingly, and tannin sacs appeared as narrow, 
elongated cells. Tannin sacs, according to Engler (5), appear abundantly 
in the pith of the Anacardiaceae and in all species of Rhus which he investi- 
gated. Pith tannin sacs are not necessarily characteristic of toxic species of 
Rhus, as Mobius (22) was unable to find them in R. vernicifera L. 

The bulk of the wood consists of simple pitted wood fibers. In trans- 
verse section they are bordered at right angles, and are assembled in rows. 
The narrower and thicker-walled cells of the fall wood contain starch; the 
wider and thinner-walled ones of spring wood appear empty. 

The pits of the tracheal vessels are exclusively simple with circular or 
elliptical outlines. The walls are relatively thick. The structure of the 
vessel wall, where it is in contact with wood parenchyma, is characteristic. 
In these places simple pits of large size are found chiefly on the vessel wall, 
and, side by side with them, either transitional or true bordered pits, but 
no separate bordered pits were noticed. The elliptical pits are transverse 
to the longitudinal axis of the vessel and parallel to one another, so that 
they remind one of scalariform perforations. 

The medullary rays are, as a rule, uniseriate; sometimes, however, they 
are biseriate. In tangential longitudinal section they are from three to 
eighteen cells high; radially their cells are joined together as are the stones 


in a wall of plane ashlar masonry. The walls of their cells are only moder- 
ately thickened, and their lumina are often filled with starch. The medul- 
lary rays are most noticeable in the lower part of the stem and in the roots. 
One small root had five primary medullary rays. 

The difference between fall and spring wood rests partly on the dis- 
similarity of the wood fiber cells and partly on that of the vessels. The 
first tracheals of spring are larger, thicker-walled, and stretched somewhat 
radially, while those toward the outer border of the annual ring are flattened 
to smaller, thicker- walled, and radial rings. The vessels in the spring wood 
are wider and more numerous, in the fall wood narrower and scarcer, as 
shown in Plate III, figure 3. The breadth of the annual rings varies. 

The inner wood is colored yellow or yellow-brown. A great deal of this 
coloring matter can be extracted with hot alcohol. This extract behaves 
similarly to the extract of the related species Rhus Cotinus L. (Cptinus cog- 
gygria Scop.) in the following treatment : an orange-yellow solution in water 
was made bright yellow by hydrochloric acid, yellow-red by ammonia, orange 
N. with alum and sodium carbonate solution, and brown N. by calcium 
chloride solution. Such a behavior by no means proves that the solutes 
from the wood of Rhus diver siloba are identical with those from R. vemicifera, 
although such may actually be the case. The coloring matter is naturally 
attached to the membrane of the wood cells, which appear golden yellow 
under the microscope and assume a brown color with caustic potash. Be- 
sides the yellow crystals, the wood cavities contain a reddish amorphous 
resinous substance which is likewise soluble in 95 percent alcohol. 

The primary cortex contains sclerosed parenchyma. 

The structure of the pericycle is characteristic. It contains many bast 
fibers, which, in transverse section, have the form of arcs whose convex 
sides are on the exterior and whose inner concave surfaces surround in each 
case a single, usually large resin duct (PI. Ill, fig. 5). 

The resin canals in the later-formed portions of the bark have a lumen 
and are arranged more or less regularly in concentric circles as heretofore 
described. The old resin canals appear to be obliterated through a kind of 
tylotic growth. On one transverse section through the bark of an old stem 
which has already thrown off the primary covering there are many resin 
canals differing in form, outline, and dimensions. The innermost are 
open and nearly circular, but usually more strictly oval in shape, stretched 
tangentially, and of larger circumference than the outer ones. The outer- 
most, particularly in old stem parts, are entirely or almost entirely obliter- 
erated through the luxuriant growth of intruding contiguous tissue. It is 
possible to observe at different heights of the same resin canal different 
states of development so that in one place it may still be open and in another 
closed. This occurrence of tyloses in the secretory ducts is similar to that 
described by Mobius (22) in Rhus vemicifera L., by Leblois in Brucea fer- 
ruginea, and by Conwentz in the intercellular canals of other plants. 


The secondary medullary rays, as already noted, are usually constituted 
of one row of cells. Where biseriate rays are found, it is sometimes noticed 
that they split apart tangentially while they remain intact radially. From 
this it would seem that adjacent cells of the two columns of the medullary 
ray are only loosely united, whereas those cells which constitute a radial 
row are more firmly attached. 

Besides what has already been said regarding the phloem, it should be 
added that the sieve tubes and their companion cells extend tangentially 
and build approximately alternating bands with the layers of phloem paren- 
chyma cells, as in the stem section of Aristolochia Sipho (27). The phloem 
apparently has but little starch, which is found deposited chiefly in the 
medullary rays. These cells also give a distinct reaction for tannin with 
ferric chloride. 

Morphology and Anatomy of the Root 
The root system in its ramifications resembles the crown, in that com- 
paratively few strong branch roots are formed which carry the fine, inter- 
laced roots. The spread of roots depends largely upon the nature of the 
soil, and upon the supply of food and water. There is a strong tendency to 
form long lateral roots, particularly in shallow soil. Propagation by layering 
is very frequently made use of naturally by the plant to insure its food supply 
and reproduction. The fine, interlacing rootlets are dark brown in color 
and are covered with fine root hairs of a lighter color. The apical tips of 
the rootlets are light yellow or colorless for several millimeters. 

As in other roots, after the secondary phloem is formed the cambium 
soon takes on a circular form in section, and behaves in the formation of 
xylem and phloem exactly as in the stem (PI. Ill, fig. 2). 

The wood of the root is less firm than that of the stem; there exist nu- 
merous large bundles, the fiber cells are less strongly thickened, the medul- 
lary rays are broader, being indeed commonly composed of two layers of cells. 

Morphology and Anatomy of the Flowers 
Rhus diver siloba is strictly dioecious, so far as my observations go. 
The male and female plants begin to bloom at about the same time. At 
Berkeley, California, but few of the flowers were open April 4, 191 5. The 
next spring the plants near the Greek Theater at Berkeley bloomed mostly 
between March 22 and May 1. In 1917 at Pasadena I noticed some male 
plants at the foot of the Mt. Wilson trail in bloom on the fifth of January. 
February 28, 191 7, the plants of both sexes were just starting to bloom in the 
Arroyo Seco, south of the Colorado Street bridge, Pasadena. In spite of 
their yellow-green color, the flower panicles are conspicuously displayed as a 
result of their size and their accumulation on the ends of the twigs. The 
presence of the staminate flowers is made very noticeable by their fragrant 
jasmine or hyacinth aroma. The pistillate flowers, on the other hand, 


have no apparent perfume. At this point it may be well to mention that 
an aromatic perfume so similar as to be perhaps identical is noticed when 
the fresh end of a freshly broken branch is smelled, and that this perfume, 
unlike that of the flowers, is not confined to the male plant, but is observed 
also in the female. The similarity between the perfume of the sap and that of 
the flower becomes more marked upon purification. The ' l aqueous solution ' ' 
as made and described in a previous paper (18) contains this more purified 
sap perfume. The panicles of the male and female flowers are somewhat 
differentiated as to location and structure. 

The flowering shoots of the male plant commonly bear as many flower 
panicles as leaves, in which case neither the highest leaves nor the lowest 
leaf develop any panicles in their axils. The lowest leaves of the flowering 
shoot soon fall off and more readily expose the flower panicles to insects, 
while the highest leaves remain and tend to protect the blossoms from the 
direct sunlight, wind, and rain. The panicles are 7 cm. long and stand 
somewhat stiffly upright at a sharp angle to the axil of the attached twig. 
The longer ones bear about a dozen side twigs of the first order, of which 
the three lowest ones are about 2 cm. long and in their turn are again richly 
branched. Toward the tip the side twigs of the first order become shorter 
and are not further branched. They are formed like a bunch of grapes, 
and the end of a panicle is likewise visibly terminated by a flower. The 
same regularity, as nearly as could be determined, appears in the arrange- 
ment of the side twigs of the first order on the panicle stem as was noticed 
in the phyllotaxy. Minute woolly hairs appear on the panicles at the 
blooming time, particularly on the bases of the panicle stem and on those 
of the side twigs. 

The flowers are placed singly on stalks from 4 to 7 mm. long, and have 
a diameter of from 5 to 7 mm. when fully opened. The flowers have 

5 calyx leaves, 5 petals, 5 stamens, and one rudimentary ovule; only by 
way of exception do 6 or 8 stamens occur, and in one flower with 6 stamens 

6 petals occurred also. 

The calyx leaves are tongue-shaped and have broad bases. They are 
about 2 mm. long and have a dark green color. 

The petals are long-elliptical in shape, narrowed at the base and at the 
point, and somewhat pointed in the front. They are 4 mm. long and in 
the middle about l 1 /^ mm. wide. When in bloom the flowers are strongly 
bent downward. The color of the petals is light green, much lighter than 
that of the calyx leaves. 

The stamens are 2 x /i mm - long. The white filaments, which are nearly 
twice as long as the anthers, shove themselves between the anther halves, 
which somewhat retreat from each other underneath. The anthers are 
introrse and are borne on upright but slightly curved filaments. 

The rudimentary ovary forms a keg-shaped pivot about 1 mm. high, 
and has 3 discernible stigmas. Between the ovary and the anthers is 
a disk, which during flowering time glistens with nectar. 


The flower, as viewed from above, is divided into 5 broad lobes, which 
stand in front of the petals and are separated by the insertion of the stamens ; 
each lobe is again slightly indented in the middle. The outer and inner ims 
of the disk are somewhat arched toward the top; from this construction a 
ring-like depression appears in the middle. 

While just as many inflorescences as leaves are found on the blossom 
shoots of the male plants, the number of panicles on the female plant is 
only about one half as great as that of the leaves. The leaves, however, 
are more numerous on the blossom shoots of the female. The number of 
leaves fluctuated between 7 and 9 in several investigations of shoots, while 
the number of panicles ranged between 3 and 5. As on the male plant, 
neither the lowest nor the highest leaves bear inflorescences in their axils but 
only the middle ones. The panicles have a length of 3 to 6 cm. They are 
not stiffly erect as in the male, but on the contrary only limply placed. The 
side twigs of the first order are up to 2.5 cm. in length, and have about as 
numerous branches, but shorter side twigs of the higher order than those of 
the male. The entire female panicle has about the same general outline 
as the male panicle. The anatomical structure of the panicle axis is essen- 
tially similar to that of the vegetative twig in the first year, and there is no 
noticeable difference in this respect between the male and the female panicle. 
Particular structures for tensile strength are not noticed in the axes of the 
fruit panicles. The stems of the pistillate flowers are not longer than 1 cm. 
and are often 5 mm. long. The flower itself is smaller than that of the male ; 
its diameter, it is true, measures about 5 mm., but the petals are less curved. 
The 5 calyx leaves are somewhat similar to those in the staminate 
flower, but slightly shorter. The 5 petals are spread out flatter and do 
not have the curled side rims. They are approximately 3 mm. long and 
1.5 mm. broad. Five stamens also occur in the pistillate flower; their 
anthers are of nearly the same length as the fertile ones of the staminate 
flower, but the filaments are about 1.5 mm. long and therefore much shorter 
than those of the male. The anthers are shrunken, of a dirty yellow color, 
with pollen absent, so that the entire pistillate flower and panicles appear 
darker. As seen from the broad side, the pistil originates in a somewhat 
compressed, egg-shaped ovary which is extended in a short style. Toward 
the top the style spreads out into three thick, brownish stigmas which are 
beset with papilli. The ovary is also to be considered as constituted by 
three carpels, of which, however, two are rudimentary so that they appear 
only in the stigmas. Between the stamens and the ovary is the disk, 
which is similar to that of the staminate flower except that it is narrower 
because of the greater expansion of the ovary. 

As far as the growth and the finer structure of the flower are concerned 
the male and female flowers show a great similarity. If one investigates 
young inflorescences on which the individual flowers are distinguishable 
as small buds, it is noticed that each flower stands in the axil of a com- 


paratively large carrying leaf which somewhat overhangs the flower. 
The outside of the bract, as well as the stigma and the axil, are covered with 
upward-bent trichomes. These trichomes are of two forms, one a single 
long bristle hair and the other a short, apparently glandular hair with a 
single-celled base and many-celled ovoid head. These hairs are similar to 
those previously described as found on the leaves and stems. Further 
developed flowers, which, with their panicles, are 2 mm. long, have a hairy 
carrying leaf longer than the panicle. The calyx leaves, the petals, and 
the stamens lie alongside each other like small enlargements and finally 
the carpels arise as wall-like growths. In this instance, in which one can 
clearly recognize the construction of the bud, the stamens are egg-shaped 
and are covered by the short petals and the longer calyx leaves. Finally 
the disk shows itself between the gynoecium and the androecium. The 
course of the vascular bundles may very clearly be recognized in the mounted 
material, as resin ducts contained in the phloem have their contents turned 
brown. In the calyx leaf, which is formed with a broad base, 5 ribs appear 
of which the middle one is the strongest and most branched. On the 
other hand, the petal, which has a small base, has only one short, weak or 
unbranched rib on each side of the strongly branched midrib. 

The disk appears in longitudinal section as a wide, somewhat sunken 
cushion. Toward the bottom its tissue is large-celled; above, on the. other 
hand, it consists of small, closely united, plasma-rich cells, of the sort com- 
mon to glandular tissues. Many small crystal clusters lie on the border 
of both tissues and in the upper, small-celled tissue, but are lacking in the 
lower, large-celled tissue. The epidermis consists of rather small polygonal 
cells and contains numerous stoma-like apertures whose guard cells are 
almost always larger than the other epidermal cells. A small space is 
found under the stoma-like opening. These openings apparently do not 
serve for gaseous interchange, but for the excretion of a glistening and 
strongly aromatic fragrant nectar whose existence has already been men- 

The development of the stamens in pistillate and staminate flowers is 
apparently similar to the time of the formation of pollen mother cells. 
In the pistillate flower no pollen grains are formed, the anthers remain 
empty, and have a shrunken appearance. The filaments of the pistillate 
flower remain as short as those of the staminate flower until the flowers open. 
The stamens naturally develop further in the latter. Pollen formation 
occurs in the anthers but shows nothing particularly noteworthy. The 
vascular bundles of the anthers contain no resin ducts, these having ended 
half-way up the filaments. The anther is also to a certain degree the only 
organ of the plant which has no resin-like or poisonous sap. It is not sur- 
prising then that the, pollen has no toxic action on the human skin (17). 
Similar observations have been made by Inui (10) on the pollen of Rhus 
vernicifera, by Warren (29) on that of R. Vernix, and by Rost and Gilg (24) 


on that of R. Toxicodendron. The pollen sacs of R. diversiloba are composed 
of two coalesced sporangia, as is common in angiosperms. Their dehiscence 
occurs by a longitudinal slit, developed where the two coalesced sporangia 
join. According to Edgeworth (4), the pollen of the Anacardiaceae is oval 
with 3 slits. The fresh pollen grains of Rhus diversiloba are ellipsoidal, 
about 1/800 sq. mm. in horizontal area, with a width 1/3 to 1/2 the length. 
The exine is roughened by minute, sharply pointed projections. When 
the pollen grains are immersed in N/4 KOH they assume a spherical form 
with no color change. In the material treated (which had been fixed in 
alcohol and xylol, stained, and mounted in balsam like the rest of the plant 
material) , the spores assumed spherical shapes or in some instances became 
rounded tetrahedrons. As is common in entomophilous plants, the pollen 
has no surfaces so modified as to permit the wind to take hold of it, of the 
nature of the bladder-like appendages of the pine pollen, etc. Whereas 
anemophilous pollen has a dry outer covering to prevent large masses of 
pollen from adhering to the flower and hindering wind transportation, the 
entomophilous pollen of Rhus diversiloba is surrounded with a sticky sub- 
stance so as to adhere to the feet and other parts of the insect. In common 
with other entomophilous flowers, R. diversiloba has perfume-secreting 
glands heretofore described which may serve to attract insects. The 
pollen itself being non- toxic and not wind-blown, the aerial transmission 
of the poison by the agency of pollen is quite out of the question. 

As in the female flower the stamens develop to a certain advanced stage, 
so the ovary develops in the male flower to the extent that an almost fully 
developed ovule is produced. Such development of an ovule in a flower 
which is functionally purely staminate, borne on a purely male plant, is a 
phenomenon which has been but rarely observed. Each ovary contains 
regularly but one ovule. The funiculus becomes curved at its apex, so 
that the body of the ovule lies against it, and, although the axis of the 
body is straight, the micropyle is directed towards the surface of origin; 
thus the funiculus appears as a ridge along one side of the body of the ovule, 
and the ovule is anatropous and consequently of the form most common 
among angiosperms. 

The ovule, in the mature female flower, fills the ovarian cavity. The 
outer integument, therefore, occupies considerable space. The micropyle 
is somewhat widely removed from the upper arching of the nucellus. The 
inner integument is widely tubular and lengthened outwardly over the 
nucellus, in which the embryo sac is again somewhat pressed back toward the 
inside so that a wider path is prepared for the pollen tube. The advantage 
of an anatropous ovule is apparent when it is remembered that the pollen 
tube advances along the wall of the ovary, and that the micropyle is thus 
brought near the wall. It is not surprising, then, that this plant with its 
efficient apparatus for fertilization should have large fruit production. 

Numerous germinating pollen grains are found on the stigmas of open 


pistillate flowers. The pollen tubes grow inside between the stigma papillae 
and pass through 4 to 6 cells of which the upper one is longest and thickest. 
On the stigmas of the staminate flowers such papillae are not formed, so 
that here no pollen grains are found. The wall of the ovary is penetrated 
by numerous vascular bundles with resin ducts which continue to the upper 
end of the pistil where the resin ducts terminate blindly with pointed ends. 
The development of the fruit, which terminates the life of the plant, 
has been taken up in another paper (19). 


1. Abrams, L. Flora of Los Angeles and vicinity. Stanford University, 191 1. 

2. Brandegee, T. S. A collection of plants from Baja California. Proc. Cal. Acad. Sci., 

ser. 2, 2: 140. 1889. 
3- • The plants of Santa Catalina Island. Zoe 1: no, 134. 1890. 

4. Edgeworth, M. P. Pollen. London, 1879. 

5. Engler, A. Uber die morphologischen Verhaltnisse und die geographische Verbreitung 

der Gattung Rhus, wie der mit ihr verwandten lebenden und ausgestorbenen Ana- 
cardiaceen. Bot. Jahrb. Syst. Pflanzengesch. u. Pflanzengeographie 1: 365-426. 
Taf. 4. 1881. 

6. Hall, H. M. A Yosemite flora, p. 151. San Francisco, 1912. 

7. Hooker, W. J. Flora boreali-americana 1: 127. London, 1833. 

8. , and Arnott, G. A. W. The botany of Captain Beechey's voyage. Part 3, p. 137. 

London, 1841. 
9. Howell, T. A flora of northwest America 1: 119. Portland, Ore., 1898. 

10. Inui, T. Ueber die Gummiharz-Gang des Lackbaumes und seiner verwandten Arten 

(Abstr.). Bot. Centralbl. 83: 352. 1900. 

11. Jadin, F. Observations sur quelques Terebinthacees. Jour, de Bot. 7: 382— 390. 1893. 
12. . Origine des secreteurs. These. Montpellier, 1888. 

13. Jepson, W. L. A flora of western and middle California. 2nd ed., p. 249. Berkeley, 


14. Leblois, A. Recherches sur l'origine et le developpement des canaux secreteurs et 

des poches secretrices. Ann. Sci. Nat. Bot. VII, 6: 247-330. 1887. 

15. Lindley, J. Rhus diver siloba. Various leaved poison oak. Edwards' Bot. Reg. 

n. ser. 18: 38. 1845. 

16. Lyon, W. S. The flora of our southwestern archipelago. II. Bot. Gaz. 11: 330 - 336. 


17. McNair, J. B. The transmission of Rhus poison from plant to person. Rhus diver- 

siloba T. and G. Jour. Infect. Dis. 19: 429-432. 1916. 
18. . The poisonous principle of poison oak. Jour. Amer. Chem. Soc. 38: 141 7-142 1. 


19. . Fats from Rhus laurina and Rhus diversiloba. Bot. Gaz. 64: 330-336. I9 1 ?* 

20. . The oxidase of Rhus diversiloba. Jour. Infect. Dis. 20: 485-498. 1917* 

21. . A study of Rhus diversiloba with special reference to its toxicity. Amer. 

Jour. Bot. 8: 127-146. 1921. 

22. Mobius, M. A. Der Japanische Lackbaum. Abhandl. Senckenberg. Naturforsch. 

Ges. 20: 201-247. 1899. 

23. Piper, C. V. Flora of the state of Washington. Contrib. U. S. Nat. Herb. 9: 384. 


24. Rost, E., and Gilg, E. Der Giftsumach, Rhus Toxicodendron L., und seine Giftwir- 

kungen. Ber Deutsch. Pharm. Ges. 22: 296-358. 1912. 

American Journal of Botany. 

Volume VIII, Plate III. 

'^rrf^V^W^^ - 5 

McNair : Morphology of Rhus. 

American Journal of Botany. 

Volume VIM, Plate IV. 

McNair : Morphology of Rhus. 


25. Schwalbe, C. On the active principle of Rhus diversiloba. Med. Rec. 63: 855. 1903. 

26. Schwalbe, K. Die giftigen Arten der Familie Rhus. Munch. Med. Wochenschr. 49: 

1616. 1902. 

27. Strasburger, E., and others. A text book of botany. Eng. transl. by H. C. Porter. 

p. 122. London, 1898. 

28. Torrey, J., and Gray, A. A flora of North America 1: 218. New York, 1838. 

29. Warren, L. E. Some observations on the pollen of poison sumach. Amer. Jour. 

Pharm. 85: 545~549- I9I3- 

Plate III 

All figures have been reduced one half in reproduction and now show magnifications 
as follows: figure 1, Xio; figure 2, Xio; figure 3, X23.3; figure 4, X470; figure 5, X91.65. 

Fig. I. Transverse section through the same stem as in figure 3. 

Fig. 2. Transverse section through a woody root. 

Fig. 3. Transverse section through a stem older than that of figure 5, showing annual 
rings with their varied formations of spring and fall growth. 

Fig. 4. Transverse section through mature leaf showing cystolith in palisade paren- 

Fig. 5. Transverse section through stem showing cork cambium; tracheal tube (P); 
pericycle with schlerenchyma cells or bast fibers and thin-walled pericycle parenchyma; 
phloem with resin duct (P); cambium (C); pith (P). 

Plate IV 

All figures have been reduced one half in reproduction and now show magnifications as 
follows: figure 1, X23.3; figure 2, X23.3; figure 3, X470; figure 4, X23.3. 

Fig. I. Transverse section through a male flower near its base, showing 5 calyx leaves 
(C) with resin ducts (P), 5 petals (P) with resin ducts (P), 5 stamens (5), and the non- 
fertile ovule (0). 

Fig. 2. Transverse section through a female flower near its apex, showing 5 calyx 
leaves (C) with resin ducts (P), 5 petals (P) with resin ducts, 5 rudimentary anthers with 
neither pollen nor resin ducts, and the fertile ovule (0) . 

Fig. 3. Transverse section through an unripe fruit near the seed, showing numerous 
crystals. Size of hexagonal crystal, 0.007 X 0.0025 mm. 

Fig. 4. Transverse section through an unripe fruit showing an abundance of resin 
ducts (RR). Diameter of largest resin duct, 0.0085 mm -